The powerful underwater volcano that erupted in the south Pacific this week has created a new island off the coast of Tonga. The eruption, about 39 miles north-west of the Tongan capital, Nuku'alofa, began on Monday, shooting rocks, steam and ash thousands of feet into the air.

Tonga's chief geologist, Kelepi Mafi, said the volcano had two vents, one on a small uninhabited island and another about 100 metres (330ft) offshore. Rock and ash spewing from the sea have filled the gap between the two vents, creating a new land mass measuring hundreds of square metres.

Last night, spectacular columns of smoke and ash were still spewing from the sea, but trade winds blew gas and steam away from the mainland and local authorities said nearby coastal villages were not at risk. Around 36 undersea volcanoes are clustered in the surrounding area.

Radio journalist George Lavaka, who visited the island with a sightseeing group, said the explosions were accompanied by a deep rumbling. "We were close to the island and there was huge explosion and the smoke was coming toward us, and people were yelling 'start the boat, start the boat'," he said. "The smoke stopped 30 metres short of the boat. We were a bit shaken actually."

Tonga, a 170-island archipelago between Australia and Tahiti, is part of the Pacific "ring of fire" - an arc of earthquake and volcanic zones stretching from Chile in South America through Alaska and down through Vanuatu to Tonga.

On Wednesday the island was shaken by a powerful undersea earthquake, prompting fears of a tsunami, but there were no reports of damage or sea surges.

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